1. Field of the Invention
The present invention is related to a poled fiber which uses the second-order optical nonlinearity, a method of fabricating the poled fiber, and a dispersion compensator which can compensate for the chromatic dispersion in a single mode fiber.
2. Description of the Related Art
Generally, the polarization P of a medium and the electric field E of applied light are related as follows:P=χ(1)E+χ(2)EE+χ(3)EEE+ . . .where χ(n) denotes the coefficient of the n-th order polarizability. If χ(2) is not equal to 0, the optical phenomenon of the second-order nonlinearity such as the second harmonic generation or the difference frequency generation is induced. In case of an optical fiber χ(2) is equal to 0 because the silica glass used for the optical fiber is the material that possesses the property of centro-symmetry. However, if a static electrical field is applied to the fiber at a high temperature, the characteristic of the second-order nonlinearity is imposed to the fiber due to the poling effect breaking the centro-symmetry. The optical fiber fabricated in this way is called a poled fiber.
The magnitude of a nonlinear electro-optic coefficient induced to the core of an optical fiber by the poling is nearly exponentially and inversely proportional to the distance between a fiber core and a electrode. If the distance between the fiber core and the electrode changes from 10 mm to 5 mm, the nonlinear electro-optic coefficient induced to the core increases about 10 times or more. However, it is not desirable that the distance between the fiber core and the electrode is too close because of large loss of light propagating through the core in the poled fiber. Accordingly, it is recommended that holes or grooves are formed appropriately as close to the core of the fiber as possible within a depth range where the transmission loss is not degraded.
By the way, even if a large second-order nonlinear coefficient is induced to the optical fiber, a phase matching condition among the traveling waves must be satisfied in order to effectively generate substantial second-order optical phenomena. Since there exists dispersion also in the optical fiber where refractive index of light varies according to the wavelength, the phase matching condition is not satisfied among the light beams with different frequencies in general situation. In order to solve this problem, several techniques, for example a phase matching technique using birefringence of the optical fiber, have been proposed. But these techniques are very picky to satisfy the condition and the wavelength range allowed by the phase matching is extremely limited. Accordingly, active control of the phase matching condition is necessary to utilize the second-order nonlinearity at required wavelength, and for this, a method of quasi-phase matching was introduced.
In U.S. Pat. No. 5,966,233 entitled “Inducing or Enhancing Electro-optic Properties in Optically Transmissive Material with Simultaneous UV Irradiation and Electric Field Application”, published on Oct. 12, 1999, an optical fiber with twin side holes has been disclosed. The optical fiber 1 with twin side holes of FIG. 13 is drawn out by making two hollow holes 4 around the core 2 of a preform using a machine tool. Thereafter, long wire electrodes 5 and 6 are inserted into the two hollow holes 4 from the opposite directions, thereby forming plane poling. According to this invention, the optical fiber with twin side holes itself performs a role of insulator between wire electrodes. Thus, a high voltage can be applied even if the distance between electrodes is short, and a long poled fiber can be simply produced at low cost. However, no concrete technique capable of forming periodic poling has been developed yet.
In U.S. Pat. No. 5,617,499 entitled “Technique for Fabrication of Poled Electro-optic Fiber Segment,” published on Apr. 1, 1997, a D-shaped fiber has been disclosed. A D-shaped fiber 10 of FIG. 14 is used after drawing directly from a D-shaped perform, or polishing the side of a normal optical fiber mechanically to fabricate it into D-shape. After the D-shaped fiber 10 is glued to a substrate 11, a poled area is formed by way of a poling process of applying high voltage to the electrodes that are formed through a series of processes such as coating of the photoresist on the plane surface, removing the coated photoresist, masking, and evaporation of fine the electrodes. The above-mentioned technique of inducing the poling to a finite length of the D-shaped fiber is well known already. Such D-shaped fibers make it possible to provide elaborate formation of electrodes and poling, excellent reproducibility, and plane and periodic poling formation. However, the D-shaped fibers formed by mechanical polishing leads to a degradation in precision, and the processes of evaporation of a fine electrode pattern onto the flat sides of the fiber is more or less complicate. Also the distance between electrodes is relatively long and the wire electrodes are not reusable.
Meanwhile, a single mode fiber, which is a transmission path in an optical communication system, has limited transmission capacity and distance because of chromatic dispersion. The chromatic dispersion of an optical fiber represents change of the refractive index of a medium according to the wavelength of light. As the refractive index of a medium determines the velocity of traveling light, the velocity difference depending on the wavelength of light increases in an optical fiber with large chromatic dispersion. If the bit velocity of an optical signal in each channel increases, the linewidth of the wavelength of the corresponding signal increases. Then component of the signal is dispersed on the time axis by chromatic dispersion, and accordingly the bit velocity for transmission is limited. In general, a single mode fiber to be used as a transmission path for optical communication has negative chromatic dispersion at 1.5 μm wavelength range in which is power loss is minimal. This implies that a portion of the optical signal with a long wavelength travels more slowly than a portion with a short wavelength. That is, in case an optical signal with a finite magnitude of wavelength linewidth travels through a single mode fiber, a portion of the optical signal with a short wavelength is temporally placed in the front part of the pulse while a portion with a long wavelength is temporally placed in the rear part of the pulse. Such chromatic dispersion is the problem to be solved in optical transmission of large capacity.
In a method of dispersion compensation by mid-span spectral inversion, a dispersion compensator is inserted in the center of an optical transmission path. Generally, if pump light with a frequency of ωp and signal light with a frequency of ωs are incident on a medium with the second-order nonlinear effect through an optical fiber coupler, light with a frequency of (ωp−ωs) can be generated according to the phase matching condition. The generated light with a frequency of ωi (ωi=ωp−ωs) is a phase-conjugated wave that has a complex conjugate relationship with the signal light of a frequency ωs. Such a phase conjugate wave of spectral inversion has the characteristic that can compensate for the phase distortion of incident light experienced on the transmission path. That is, if we see before and after the mid-span spectral inversion, since a short wavelength of the signal light is phase-inverted to a long wavelength of the generated light and vice versa, the chromatic dispersion experienced in the first half part is symmetrically compensated if the generated light again travels through the remaining half part of the transmission path.
In Korean Patent Publication No. 2001-11093, entitled “Chromatic Dispersion Compensator Using Poled Fibers,” published on Feb. 15, 2001, a chromatic dispersion compensator using a poled fiber has been disclosed. The chromatic dispersion compensator is shown in FIG. 11. A dispersion compensator 1100 of FIG. 11 includes a periodic poled fiber 1160, amplifiers 1130 and 1150, an optical fiber coupler 1140, and a filter 1170. The pole fiber 1160 is placed at the center of a transmission region. If the frequency ωp of pump light is similar to the frequency ωs of signal light used for communications, that is, if ωp≅ωs, the dispersion compensator 1100 in FIG. 11 can be used. The signal light dispersed while traveling through a transmission path is coupled at the optical fiber coupler 1140 along with pump light amplified by the amplifier 1130, and is put into the periodic poled fiber 1160 via the amplifier 1150. The periodic poled fiber 1160 generates a second-order harmonic wave with 2ωp from the phenomenon of the second harmonic generation of the pump light (2ωp=ωp+ωp), which corresponds to ½ the wavelength of the pump light. Then it at the same time converts the second-order harmonic wave, which is due to the phenomenon of the differential frequency generation (ωi=2ωp−ωs) between the second-order harmonic wave and the signal light, into a phase conjugate wave of which the dispersion can be compensated. Only the idler wave (ωi) that is mid-span spectrally inverted at the periodic poled fiber passes through the filter 1170. The output wave from the filter 1170 travels along the transmission path. However, the chromatic dispersion compensator disclosed in Korean Patent Publication No. 2001-11093 can only be used in case of the signal and the pump lights with similar wavelengths, and cannot be practically used in case the wavelength of pump light is ½ of signal light.